This invention relates to a hot gas engine and, in particular, to controlling the operation of a Stirling engine suitable for use in an automotive application.
The Stirling or hot gas engine cycle is well known in the art. A two-cylinder Stirling engine is described in U.S. Pat. Nos. 3,984,983 and 3,999,388, while a four cylinder engine is further described in U.S. Pat. Nos. 3,914,940 and 4,474,003. The Stirling engine is durable, clean burning, and exhibits relatively high efficiency when compared to the more conventional internal combustion engine. The Stirling engine, however, is relatively slow to respond to changes in power demands and thus difficult to adapt for use in motor vehicles where engine acceleration and deceleration must be rapid. Recently, efforts have been undertaken to improve the response time of the Stirling engine so that it might be better suited for use in motor vehicles.
In the hot gas engine, one method of regulating the engine power output is by changing the pressure of a working gas, favorably a light gas such as helium or hydrogen, contained within the engine. To increase the engine's output power, the internal gas pressure is increased by adding gas to the engine from an external supply reservoir. To decrease the engine pressure, gas is typically pumped from the engine back to the supply reservoir using a compressor.
Single acting or double acting compressors are generally used to pump down the hot gas engine. In either case, the compressor has a single capacity. In order to attain a satisfactory idle pressure, which is usually about four megapascals, or MPa, the capacity of the compressor must be relatively low. As a consequency, the pump down rate of the engine is correspondingly slow, and the time required to bring the engine pressure from some high operating value to idle can be far too long for efficient use in an automotive application. The portion of the engine gas that cannot be pumped by the limited capacity compressor is short-circuited back to the engine. The energy contained in the short-circuited gas is dissipated in the engine and represents lost power, thereby reducing engine efficiency. This type of efficiency penalty can be relatively large and can only be minimized by increasing the pump down rate.
The system of U.S. Pat. No. 3,782,119 does not include a compressor. The natural pressure wave of the Stirling power cycle is utilized to increase or decrease the gas pressure. A series of supply tanks are provided, each of which is maintained at a different pressure. Through use of a control valve, one or more of the tanks can be connected to the engine to raise the engine pressure to some desired level. During pressure reduction, the engine working gas is bled back into tanks, again selectively sequencing the control valves. The valving scheme, by necessity, must be rather complex and maintaining close control over the tank pressures is sometimes difficult. For smooth operation, care must be taken that the appropriate valves are opened and closed at the proper times. Furthermore, the response of the engine without the aid of a compressor is relatively slow.
In pumping down to idle pressure, the compressor can have a working gas pressure at the output side many times what it is at the suction side. However, for optimum operation, this ratio of output to suction should not exceed about four for a single stage compressor. Even so, the same compressor must still be able to deliver pump down working gas to the storage tank at a heavy motor load condition, at idle, and over a broad range of power load conditions in between.